This application pertains to a method and apparatus for frustrating the phenomenon of total internal refection in a continuously variable, easily controllable manner.
It is well known that light travels at different speeds in different materials. The change of speed results in refraction. The relative refractive index between two materials is given by the speed of an incident light ray divided by the speed of the refracted ray. If the relative refractive index is less than one, as in the case when light passes from glass block to air, then a light ray will be refracted towards the surface. Angles of incidence and reflection are normally measured from a direction normal to the interface. At a particular angle of incidence xe2x80x9cixe2x80x9d, the refraction angle xe2x80x9crxe2x80x9d becomes 90xc2x0 as the light runs along the block""s surface. The critical angle xe2x80x9cixe2x80x9d can be calculated, as sin i=relative refractive index. If xe2x80x9cixe2x80x9d is made even larger, then all of the light is reflected back inside the glass block and none escapes from the block. This is called total internal reflection. Because refraction only occurs when light changes speed, it is perhaps not surprising that the incident radiation emerges slightly before being totally internally reflected, and hence a slight penetration (roughly one micron) of the interface, called xe2x80x9cevanescent wave penetrationxe2x80x9d occurs. By interfering with (i.e. scattering and/or absorbing) the evanescent wave one may prevent (i.e. xe2x80x9cfrustratexe2x80x9d) the total internal reflection phenomenon.
In a number of applications, it is desirable to controllably frustrate the phenomenon of total internal reflection. For example, if total internal reflection is occurring at an interface xe2x80x9cIxe2x80x9d as shown in FIG. 1A, the extent of such reflection can be reduced by placing a dielectric material xe2x80x9cDxe2x80x9d close to interface I, such that dielectric D interacts with the evanescent wave penetrating beyond interface I, as shown in FIGS. 1B, 1C, and 1D, in which the extent of frustration of total internal reflection is gradually increased, culminating in complete frustration (FIG. 1D).
It is desirable that dielectric D be an elastomeric material. Inevitably, at least some foreign particles xe2x80x9cPxe2x80x9d (FIG. 2A) are trapped between dielectric D and interface I; and/or, the opposing surfaces of dielectric D and interface I have at least some dimensional imperfections xe2x80x9cXxe2x80x9d (FIG. 2B) which prevent attainment of a high degree of surface flatness over substantial opposing areas of both surfaces. Such foreign particles, or such surface imperfections, or both, can prevent attainment of xe2x80x9coptical contactxe2x80x9d between dielectric D and interface I. Optical contact brings dielectric D substantially closer than one micron to interface I, thereby scattering and/or absorbing the evanescent wave adjacent interface I, thus preventing the capability of interface I to totally internally reflect incident light rays. If dielectric D is formed of an elastomeric material, the aforementioned adverse effects of such foreign particles and/or surface imperfections are localized, thereby substantially eliminating their impact on attainment of the desired optical contact. More particularly, as seen in FIGS. 2C and 2D, the elastomeric nature of dielectric D allows dielectric D to closely conform itself around foreign particle P and around surface imperfection X, such that optical contact is attained between dielectric D and interface I except at points very close to foreign particle P and around surface imperfection X. Since such points typically comprise only a very small fraction of the opposing surface areas of dielectric D and interface I, sufficiently substantial optical contact is attained to facilitate frustration of total internal reflection as described above.
Elastomeric materials vary considerably in surface tack, but virtually all are too tacky to be practical for this application without modification. This is because most elastomeric materials are sufficiently soft and have enough surface energy that the material can deform into intimate xe2x80x9catomic contactxe2x80x9d with the atomic scale structure present at any surface. The resulting Van der Waals bonding is sufficient to make it difficult to remove the material from the surface.
It is desirable to provide a means for controlling frustration of total internal reflection by varying an interfacial pressure applied between dielectric D and interface I; and, in general, it is desirable to minimize the applied pressure. The aforementioned Van der Waals bonding can require negative pressures of order 104 Pascals for release, which is desirably reduced. Further, it is desirable to separate dielectric D and interface I by an amount exceeding the evanescent wave zone when the applied pressure is removed. The present invention addresses these desires.
The invention provides an optical switch for controllably switching an interface between a reflective state in which light incident upon the interface undergoes total internal reflection and a non-reflective state in which total internal reflection is prevented at the interface. In one embodiment, the switch incorporates a preferably elastomeric dielectric having a stiffened surface portion. A separator is positioned between the interface and the stiffened surface portion to maintain a gap there-between. Electrodes are applied to the interface and stiffened surface portion respectively. A voltage source controllably applies a variable voltage potential between the electrodes. Application of a voltage potential between the electrodes moves the stiffened surface portion into optical contact with the interface, producing the non-reflective state at the interface. In the absence of a voltage potential between the electrodes the separator moves the stiffened surface portion away from optical contact with the interface, producing the reflective state at the interface.
The separator may be a plurality of stand-offs provided at spaced intervals between the interface and the stiffened surface portion to maintain the gap at about 1 micron in the absence of a voltage potential between the electrodes. Advantageously, the stand-offs are an integral part of the interface.
The dielectric""s surface may be stiffened by applying to it a thin film material having a Young""s Modulus value substantially less than the dielectric""s Young""s Modulus value. Alternatively, and to better enable the dielectric""s surface to flex in the vicinity of the standoffs, the dielectric""s surface may be stiffened by applying a thin layer of hard particles thereto.
In another embodiment, the optical switch incorporates a cell containing a fluid. One side of the cell forms the interface upon which light is incident. A membrane is suspended in the fluid. One pair of electrodes is applied to opposite sides of the membrane; and, another electrode pair is applied to the side of the cell forming the interface and to the cell""s opposite side. A variable voltage potential is applied between selected ones of the electrodes. Application of the voltage potential between selected ones of the membrane and cell electrodes moves the membrane into optical contact with the interface, producing the non-reflective state at the interface. Application of the voltage potential between other selected ones of the membrane and cell electrodes moves the membrane away from optical contact with the interface, producing the reflective state at the interface.